242 THE EVOLUTION OF THE METAZOA 



Thus the typical alternation of generations, medusa: polyp, 

 had evolved twice, and each time from its own root. In spite 

 of the great similarities that can be observed in the forms and 

 basic organisations of scyphomedusae and hydromedusae— a 

 fact which can be explained as due to the similarities that exist 

 in scyphopolyps and hydropolyps — there exist considerable 

 differences between the two medusa forms especially as regards 

 the way how they are developed. With the exception of the 

 secondarily acquired hypogenesis, we find that all the scypho- 

 medusae develop without any exception from scyphopolyps 

 by way of an unequal transverse division, while all the hydro - 

 medusae develop equally, without any exception, by way of 

 budding. The division must be considered as the more primi- 

 tive form of the asexual reproduction, and budding as its 

 more advanced form. I wish to mention in passing that as 

 early as in the Anthozoa we can find in the higher subtype of 

 the Octactiniaria budding only as the form of their vegetative 

 reproduction while division prevails in the Hexactiniaria. In 

 the scyphozoan polyps budding prevails, yet transverse divi- 

 sion has been preserved in the formation of the medusa 

 generation. 



It should be mentioned in passing that it is already time that 

 the hydroidologists as well as other zoologists adhere strictly 

 to the names Thecata (after Fleming, 1828), and Athecata 

 (after Hincks, 1868) when they speak of the two groups 

 of Hydroidea; they should avoid the awkward names Calypto- 

 blastea (or even Calyptoblastiques), Thecaphora, or the too 

 narrowly interpreted Campanulariidea (Stechow, 1922), Gym- 

 noblastea (Gymnoblastiques with the addition Hydroida), 

 Tubulariidea (Stechow, 1922). The double names : Thecaphorae- 

 Leptomedusae and Athecatae-Anthomedusae (after Broch, 

 1924) can also be dispensed with. 



Capitata (Kuhn, 1914; Coryninea, according to Stechow, 

 1922) have been placed, within the group Athecata, in front of 

 Filifera or Bougainvillinea; this has been so because it has 

 been believed that the actinula with tentacles that had thickened 



